Devices responsive to linear acceleration perform essential sensing functions in a wide variety of systems. As performance requirements and available technology have advanced, the demand has increased for sensors characterized by much improved sensitivity, stability, accuracy, linearity of response, reliability, and ruggedness, in addition to fast reaction time, minimum cross-coupling, small size, and low cost. Implicit in the stability, sensitivity, and linearity requirements is a requirement that precision be maintained over a wide temperature range. The present state of the art is such that it has been difficult to achieve improvements in all of the foregoing characteristics simultaneously, or, in some instances, even to achieve improvement in one characteristic without adversely affecting another. Nevertheless, requirements exist, particularly in aircraft navigation and missile guidance systems, for an acceleration sensor with superior performance in all the noted areas.
In my co-pending patent application, Ser. No. 486,144, filed April 18, 1983, and assigned to the assignee of the present application, I disclose an accelerometer structure which minimizes bias instability, and reduces cross-coupling errors, by use of a sensing capsule including as a pendulum a sheet of silicon, supported at flexures across opposites faces of which are implanted strain sensitive resistors, so that as the pendulum moves in response to accelerations, the sensors detect the actual departure of the system from its physical null.
In use, the acceleration being sensed is applied in a direction which torques the pendulum about its flexures, so that the sensors on one surface increase in resistance when those on the opposite surface decrease in resistance, and vice versa. The resistances are connected in bridge circuits and the bridge outputs are responsive to the actual stresses in the flexures, and hence to the actual displacement of the flexures from mechanical null.
If the device is subject to accelerations orthogonal to that intended, the upper and lower resistors of each flexure are varied in the same sense, and no bridge unbalance occurs. Thus the system operation is made substantially independent of accelerations orthogonal to that desired, and cross-coupling is reduced.
The system described above has a disadvantage in that it requires diffusion or implantation of strain sensitive resistors at particular places on both surfaces of the sheet of silicon forming the pendulum. Processing on both surfaces of the material is difficult and expensive due to the necessity of careful registration between the upper and lower masks, and the problem of protecting one surface while the other is being worked on.